I work long hours and am self employed so I understand the affect this can have on a relationship dynamic with DC and DP.
I would hate to know my DC would say 'yay' if I wasn't able to go to something or that they were avoiding the area when I am around.
I don't believe anyone is genuinely happy being grumpy all the time and often use it as a defence mechanism.
I can't see that he is happy. No one is happy making others unhappy, it might give him a sense of control because it shows you 'care' about the relationship but the only positive in that is he gets to be with his son, in his house, keeps his earnings or whatever other meaningless crap he is worried about losing.
Knowing how his relationship with his son may be improved/protected could mean he is comfortable with a break up.
He may even start to be more focussed on making his time with his son enjoyable for both knowing his own time is protected
I know MN would refer to this as Disney Dad and how it is a negative in an ex's relationship with DC, but I really think this is better for the DC than Grumpy Dad etc and MN sometimes loses the context that fathers love their kids too and feel the seperatiin deeply.
Disney dad can be about making the best of what time you have with dc not consciously undermining the ex.
I think your DS will be better off with Mr Grumpy's version of Disney Dad.
FWIW, the grumpiest man I know is happiest on his own with occasional relationships. It allows him to get what he wants without being grumpy with others and he admits it. He is however a great father and really supportive of his ex - financially, emotionally etc. I don't think it is a side him of them would have see had they stayed together